Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter within the central nervous system (CNS). GABAA receptors are ligand gated ion channels that are made up from a large range of different subunits (α1-6, β1-3, γ1-3, δ, ε, π, and θ). Each receptor complex comprises five subunits, with the dominant in vivo combination thought to be 2α2β1γ. Several therapeutic agents exert their effects by modulating this receptor complex, but adverse effects, particularly sedation, are common and, in part, a consequence of poor subunit selectivity. The existence of a large number of different GABA-A receptors resulting from subunit heterogeneity indicates that there are excellent prospects for developing more selective drugs for the treatment of CNS disorders with reduced side effects. To date, the majority of the ligands that have been identified bind to α subunits that are sensitive to classical benzodiazepines, namely α1, α2, α3 and α5. Without exception, these ligands bind allosterically to the receptor, rather than by occupying the orthosteric (GABA) site and can exert a range of pharmacological activities including agonists, antagonists, partial agonists, and inverse agonists.
Agents that bind or interact with the modulatory sites on the GABAA receptor complex, such as, the benzodiazepine receptor, can have either enhancing effect on the action of GABA, i.e. a positive modulatory effect of the receptor (agonists, partial agonists), an attenuating effect on the action of GABA, i.e. negative modulation of the receptor (inverse agonists, partial inverse agonists), or they can block the effect of both agonists and inverse agonists by competitive block (antagonists or ligands without intrinsic activity).
The binding of the compounds of the current invention at or near the benzodiazepine receptor complex suggests that the compounds of the invention may facilitate the inhibitory action of the neurotransmitter GABA and therefore its synaptic effects. As stated above, benzodiazepine receptors, which can be located both within the central nervous system and peripherally (e.g., in the endocrine system), are comprised of macromolecular complexes characterized by sites for binding of the benzodiazepines and GABA. The benzodiazepine receptor complex is further associated with, and interacts with, a transmembrane channel for chloride ion transport. The effect of the compounds of the current inventions' interaction with the benzodiazepine receptor/GABA receptor/chloride channel complex is to cause GABA to inhibit cerebral neuronal discharge, presumably by increasing membrane conductance of chloride ion, thus stabilizing membrane potentials and dampening excitatory input. (See Meldrum, B. S. Brit. J. Clin. Pharm. 27 (suppl. 1), 3S-11S (1989)). Through mediation of this process, the compounds of the current invention may be useful in treating anxiety disorders and a number of other conditions in which GABA is believed to exert a physiologic role. These conditions include psychiatric disorders, convulsive disorders, aggressive behavior, muscle spasms or tensing, depressive or bipolar disorders, cognitive disorders, sleeping disorders, neurodegenerative eye diseases, neurodegeneration, pain, emesis, or eating disorders. The present invention also includes methods for treating the above-described conditions or disorders in a human by administering the compounds of the invention to the human.
Agonists generally produce muscle relaxant, hypnotic, sedative, anxiolytic, and/or anticonvulsant effects, while inverse agonists produce proconvulsant, antiinebriant, and anxiogenic effects. Compounds with anxiolytic effects, but without or with reduced muscle relaxant, hypnotic and sedative effects are characterized as partial agonists. Partial inverse agonists are considered to be useful as cognition enhancers.